The present invention relates to a zoom lens suitable for, for example, a projection lens used in an image projection apparatus (projector).
Projectors require that a change in projection distance does not cause a significant change in quality of a projected image, that is, causes only small variation in performance of a projection lens.
Further, in the projector, color light components emerging from image-forming elements such as liquid crystal panels for R, G, and B are combined by a color-combining optical system to be introduced to the projection lens, so that the projection lens needs to have a long back focus. Therefore, the projection lens often employs a so-called retrofocus type lens configuration in which a strong negative refractive power is disposed closer to a projection surface (screen) than an aperture stop.
However, the retrofocus type lens has a feature that curvature of field due to the projection distance change is easily caused mainly by asymmetry of the lens, and therefore it is difficult to suppress the variation in performance of the projection lens due to the projection distance change. Moreover, an increase of a field angle of the retrofocus type lens for shortening the projection distance makes the curvature of field more obvious. Further, an image-forming element with high resolution decreases a permissive range of the curvature of field due to the projection distance change.
A method called as floating has been conventionally used for reducing the curvature of field due to the projection distance change. In the floating, plural lens units are simultaneously moved so as to mutually correct aberrations such as the curvature of field generated in the respective lens units. In particular, a lens configuration, in which a first lens unit that is disposed closest to a magnification side and does not contribute to variation of magnification is divided into two lens sub-units to perform the floating, has a merit that no variation of focus position is caused by the variation of magnification.
Such floating performed using the first lens unit divided into two lens sub-units has been disclosed in, for example, Japanese Patent Laid-Open No. 2002-35771 (corresponding to U.S. Pat. No. 6,580,564).
In addition, a so-called inner focus type zoom lens has been disclosed in Japanese Patent No. 3263835 and Japanese Patent Laid-Open No. 2001-124991 (corresponding to U.S. Pat. No. 6,686,988) in which a most-magnification side lens sub-unit is fixed and a reduction side lens sub-unit disposed next to the most-magnification side lens sub-unit on a reduction side is moved for focusing, which, however, is hard to be classified as the floating. In the inner focus type zoom lens, the most-magnification side lens sub-unit has a negative refractive power and the reduction side lens sub-unit moved for focusing has a positive refractive power.
Further, a zoom lens has been disclosed in Japanese Patent Laid-Open No. 2006-234893 (corresponding to U.S. Pat. No. 7,190,528) in which a lens sub-unit having a negative refractive power and being disposed closer to the magnification side than a fixed lens sub-unit having a positive refractive power is moved for focusing.
The zoom lens disclosed in Japanese Patent Laid-Open No. 2002-35771 has a negative refractive power as a whole, and therein the floating is performed by dividing the first lens unit that is unmoved for the variation of magnification into a first-A lens sub-unit and a first-B lens sub-unit respectively having a negative refractive power. However, the floating in a negative-negative type zoom lens is not suitable for miniaturization thereof because a lens movement amount is increased compared to a case where the focusing is performed by moving the entire first lens unit.
The zoom lens disclosed in Japanese Patent No. 3263835 does not have a sufficient effect to correct variation of an image plane because a ratio of the refractive power of the first-B lens sub-unit to that of the entire first lens unit is large. In addition, the zoom lens disclosed therein is not suitable for a projection lens that needs to be telecentric on the reduction side because a most-reduction side lens unit thereof has a negative refractive power.
The zoom lens disclosed in Japanese Patent Laid-Open No. 2001-124991 has an extremely small effective image circle, is insusceptible to the curvature of field, and includes the first-B lens sub-unit having an extremely small refractive power. Thus, the zoom lens is a zoom lens having an object different from that of the present invention.
The zoom lens disclosed in Japanese Patent Laid-Open No. 2006-234893 improves the curvature of field to some extent compared to the case where the entire first lens unit is moved. However, it cannot provide a sufficient effect to remove the variation of the curvature of field.